In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, for example, an image is generally formed on a surface of a sheet (the term “sheet” is herein defined to include a paper sheet, a plastic film such as an OHP film and the like, and this definition is effective in the following description) through the following process steps.
First, a surface of a photoreceptor body having photoconductivity is evenly electrically charged and, in this state, exposed to light, whereby an electrostatic latent image corresponding to an image to be formed on the surface of the sheet is formed on the surface of the photoreceptor body (charging step and exposing step).
Then, a toner (minute color particles) preliminarily electrically charged at a predetermined potential is brought into contact with the surface of the photoreceptor body. Thus, the toner selectively adheres to the surface of the photoreceptor body according to the potential pattern of the electrostatic latent image, whereby the electrostatic latent image is developed into a toner image (developing step).
Subsequently, the toner image is transferred onto the surface of the sheet (transfer step), and fixed to the surface of the sheet (fixing step). Thus, the image is formed on the surface of the sheet.
In the transfer step, the toner image formed on the surface of the photoreceptor body may be directly transferred to the surface of the sheet, or may be once transferred to a surface of an image carrier (first transfer step) and then transferred to the surface of the sheet (second transfer step).
A transfer roller formed from an electrically conductive rubber composition and having a predetermined roller resistance is used for transferring the toner image from the surface of the photoreceptor body to the surface of the sheet in the transfer step, for transferring the toner image from the surface of the photoreceptor body to the surface of the image carrier in the first transfer step, or for transferring the toner image from the surface of the image carrier to the surface of the sheet in the second transfer step.
In the transfer step for the direct transfer, for example, a predetermined transfer voltage is applied between the photoreceptor body and the transfer roller kept in press contact with each other with a predetermined pressing force and, in this state, the sheet is passed between the photoreceptor body and the transfer roller, whereby the toner image formed on the surface of the photoreceptor body is transferred to the surface of the sheet.
Lately, transfer rollers to be incorporated in general-purpose laser printers and the like particularly for use in developing countries tend to be required to have a simplified construction so as to be produced at lower costs possibly by using versatile materials.
To meet the requirement, transfer rollers having a porous structure are widely used. The porous structure requires a reduced amount of a material to reduce material costs, and has a reduced weight to reduce transportation costs. The porous structure imparts the transfer roller with proper flexibility even if a plasticizer is not blended or blended in a reduced amount in the material.
For production of the transfer roller of the porous structure, it is preferred to employ the following continuous production method, for example, in order to improve the productivity of the transfer roller to further reduce the production costs of the transfer roller.
That is, the transfer roller can be continuously produced at higher productivity by extruding an electrically conductive rubber composition into an elongated tubular body by means of an extruder, continuously feeding out the extruded tubular body in the elongated state without cutting the tubular body to pass the tubular body through a continuous crosslinking apparatus including a microwave crosslinking device and a hot air crosslinking device for continuous foaming and crosslinking, and then cutting the resulting tubular body to a predetermined length.
It is preferred to use an expensive ion conductive rubber such as an epichlorohydrin rubber in combination with a crosslinkable rubber as a rubber component for the electrically conductive rubber composition in order to reduce the material costs and further reduce the production costs of the transfer roller.
A typical example of the crosslinkable rubber is an acrylonitrile butadiene rubber (NBR). In order to further reduce the production costs of the transfer roller to meet the aforementioned requirement, it is more preferred to use a styrene butadiene rubber (SBR) and an ethylene propylene diene rubber (EPDM) in combination as the crosslinkable rubber.
The combinational use of the SBR and the EPDM makes it possible to impart the transfer roller with proper ozone resistance while further reducing the material costs.
That is, it is possible to reduce the proportion of the expensive ion conductive rubber required for the production of the transfer roller having a comparable roller resistance. In addition, the SBR is more versatile and less costly than the NBR, so that the material costs can be further reduced.
However, the SBR is insufficient in resistance to ozone to be generated inside the laser printer or the like, i.e., has poorer ozone resistance. Therefore, the SBR is used in combination with the EPDM.
The EPDM per se does not only have excellent ozone resistance, but also serves to suppress degradation of the SBR due to ozone. This improves the ozone resistance of the transfer roller.
An azodicarbonamide (H2NOCN═NCONH2, ADCA) forming agent and an urea foaming assisting agent are typically used in combination as a foaming component.
However, ADCA is decomposed through molecular vibration heating by irradiation with microwaves to produce cyanic acid, isocyanic acid, cyamelide, cyanuric acid and isocyanuric acid and the like as decomposition residue.
Urea is also decomposed through molecular vibration heating by irradiation with microwaves to produce cyanic acid and the like as decomposition residue.
These decomposition residues are liable to impair the electrical conductivity of the ion conductive rubber, to inhibit the crosslinking reaction of the rubber component and to contaminate the photoreceptor body.
To cope with this, Patent Literatures 1 and 2 propose that an additive such as sodium hydrogen carbonate (sodium bicarbonate) thermally decomposable to produce water is used in combination with ADCA and urea.
In the presence of the water produced by the decomposition of sodium hydrogen carbonate, it is possible to convert the decomposition residues into substances free from the impairment of the electrical conductivity, the inhibition of the crosslinking reaction and the contamination of the photoreceptor body through the molecular vibration heating by the irradiation with the microwaves.